Optical waveguide devices typically employ lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3) substrates with a defined waveguide path, and bias electrodes formed over the substrate. They may also include a buffer oxide between the electrodes and substrate. An electrical bias supplied to the electrodes alters the index of refraction of the waveguide path so that the device may be employed as a high speed optical switch, modulator, attentuator or other type of optical device.
For proper operation of such devices, it is highly desirable that a dc electric field bias be maintained in the waveguide for extended times and varying temperatures. Therefore, long term bias stability is a critical concern. Most approaches to providing bias stability have involved reducing OH or proton concentration on the lithium niobate substrate (Nagata et al "Improved Long Term Drift in OH-Reduced Lithium Niobate Optical Intensity Modulators", Optics & Photonics News, vol. 7, Engineering & Laboratory Notes (1996) and Koide, et al "Prevention of Thermal Degradation by Using Dehydrated LiNbO.sub.3 Crystal" Jpn. J. App Phy., vol. 33, pp. L957-L958 (1994)), removal of a damaged layer from the surface of the lithium niobate substrate (Minikata et al, "DC Drift Free Ti Diffused LiNbO.sub.3 Optical Modulators" Bulletin of the Research Institute of Electronics, vol. 30, pp. 209-212 (1995)), or modification of the composition of the buffer layer (EP 0 553 568 A1). Such approaches appear effective in eliminating long term drift. However, pyroelectric material, such as lithium niobate and lithium tantalate, is also sensitive to temperature variations. That is, temperature variations induce charge build-up at the surfaces of the substrate, and this build-up generally requires an increasing bias to achieve the desired modulation. The above solutions do not deal with bias instability due to such temperature variations.
One approach to the instability problem is to provide a semi-insulation charge dissipation layer, such as partially oxidized polysilicon or Ti/Si/N, over the substrate. We have found, however, that the effectiveness of this charge dissipation layer can degrade under bias over a period of time, thus limiting its effectiveness.
It has also been suggested that lithium niobate material used for surface acoustic wave devices can be treated by heating in a forming gas (10 percent H.sub.2, 90 percent N.sub.2) for one hour at temperatures in the range 400-750 deg C. (see Standifer et al "Chemically Reduced Lithium Niobate Single Crystals . . . " IEEE Frequency Control Symposium, May 27-29, 1998, pp. 470 to 472.) This is done to suppress sparking during processing and operation as well as reduce optical reflections during photolithographic processing of the device.